When Melbourne Flight Service told Frederick Valentich there was “no known traffic” near Cape Otway, many took it as proof he was alone in the sky. The truth is more complicated.

In 1978, radar coverage over south-western Victoria was patchy at best. Below certain altitudes and beyond certain ranges, an aircraft, or anything else, could operate entirely unseen by ground controllers. “No known traffic” meant precisely that: nothing known.

Understanding the limits of surveillance technology is essential to assessing any aerial encounter. An empty radar screen has never been the same thing as an empty sky.